What is the ruling on praying for marriage to another righteous man, given that I previously prayed to Allah for my husband to remain my husband in this world and the Hereafter, and a divorce has occurred between us?

It is permissible for you to supplicate to Allah to grant you a righteous husband after your separation from your first husband, and your previous supplication to Allah that your first husband be your companion in Paradise does not prevent you from doing so.